Control system for air quality and temperature conditioning unit with high capacity filter bypass

ABSTRACT

An air conditioning unit is provided with a high capacity filtration system for removing pollutants from supply air, typically return air which has been recirculated from the conditioned area. A filter bypass passage and suitable flow control devices are provided so that supply air, typically fresh outdoor air, may be routed around the filtration system when minimal or no filtration is required. An air quality detector samples the air quality and positions the flow control devices to effect the most cost efficient operation of the air handling unit under the given operating conditions.

BACKGROUND OF THE INVENTION

This invention relates in general to air conditioning and heating unitsfor supplying conditioned air to the main duct work of large and smallbuildings and, more particularly, to a unit which contains high capacityfilters for removing pollutants from the conditioned air.

Air conditioning and heating units are used to supply temperatureconditioned air to the main duct work of buildings. One or more largefans within the unit are used to draw or blow supply air therethroughduring which the heating or cooling coils in the unit providetemperature conditioning of the supply air. The supply air may compriseair which is recirculated from the building (i.e., return air), eitheralone or in combination with fresh air drawn from outside of thebuilding. After the supply air has been heated or cooled, it is thendischarged by the fan into the associated air distribution system. Inother cases, the fan blows the air through the heating and cooling coilsand then into the distribution system.

In addition to providing temperature conditioning of the air, units ofthis type must also maintain an acceptable level of air quality withinthe conditioned space. The levels of pollutants such as volatile organiccompound and cigarette smoke which are generated within the building aretypically controlled by a high capacity filtration system which removesairborne contaminants from the recirculated air. High capacityfiltration devices such as mechanical filters and electronic andadsorptive devices are effective at removing undesired pollutants, butalso typically cause a significant pressure drop across the filters. Inorder to compensate for the pressure loss, more fan energy must beutilized, thus significantly increasing the operating costs of thesystem. Replacement costs of high efficiency filters are higher thanreplacement costs of lower efficiency media, so action to extend thelife of the high efficiency filter will remove system operating costs.The use of these extensive filtration systems has also become moreprevalent as a result of increasingly stringent air qualityrequirements.

For example, in 1989 the Professional Society for Air Conditioning(ASHRAE) changed its ventilation standard by increasing the recommendedamount of outside air to be incorporated into a ventilation system.ASHRAE increased the recommended amount by a factor of 4, from 5 cfm(cubic feet per minute) per person to 20 cfm per person for officebuildings and from 5 cfm per person to 15 cfm per person in schools.Generally, a conditioning system consumes more energy when processingoutside air, such as when changing its temperature, as compared to theenergy consumed when processing return air, such as during filtration.Thus, increasing the recommended amount of outside air for use in aventilation system, ASHRAE similarly increased the energy consumed bythe ventilation system. For instance, an air conditioning system for anoffice building consumes between 10% to 15% more energy when processing20 cfm per person of outside air, as compared to the energy consumedwhen processing 5 cfm per person.

The ASHRAE standard provides an alternative to increasing the amount ofoutside air. This alternative is referred to as the "indoor air qualityprocedure" (IAQ procedure) and is referred to as "demand controlventilation" (DCV). The IAQ procedure may usually substantially reducethe energy consumption of the conditioning system by allowing forvariations in the percentage of outside and return air to be utilized.The IAQ procedure sets maximum contaminant levels acceptable within theoccupied space. The DCV system is merely required to maintaincontaminant levels within these acceptable maximums without mandatingthe use of 20 cfm per person of outside air.

The ASHRAE standard has been adopted by many states and nationalbuilding codes. Presently, additional government agencies are becomingmore involved in the regulation of indoor air quality within commercialbuildings, and thus interest has increased in the IAQ procedurerecommended by ASHRAE.

However, presently a conditioning system does not exist which adequatelyfollows the IAQ procedure to maintain acceptable contaminant levelswithin a commercial building while minimizing the energy consumption ofthe conditioning system. In addition, previous systems which haveattempted to comply with the IAQ standards have unnecessarily limitedthe life of the filter media since these foregoing systems drawexcessive amounts of return and outside air through the high efficiencyfilters even when not required by the air quality. This excessive useshortens the high efficiency filter's life and increases theconditioning systems energy consumption. Such energy consumption resultsfrom the fact that high efficiency filters cause a substantial pressuredrop within air passed therethrough. This pressure drop must becompensated for by the fans within the conditioning system, therebyconsuming excess energy.

Moreover, often, fresh outdoor air which is drawn into the unit has arelatively high air quality and does not require the extensive filteringnecessary with recirculated air. There are also occasions when therecirculated or return air is of sufficient quality to achieve thedesired air quality standards without filtration. However, past systemshave not been designed to bypass the filters when the incoming outsideand/or return air has acceptable quality. Thus, the extensive filtrationsystems which are required to achieve necessary filtration at peakfiltration loads are also used at non-peak times and thus causesignificant unnecessary operating costs at non-peak times.

Finally, past systems have assumed outside air to be good quality andhave used more outside air to dilute indoor contaminates without regardto outdoor air quality nor with regard to the cost of heating or coolingthis additional outside air. The amount of outside air depends upon thequality thereof and upon the quality of the return air. Thus, maximizingthe use of uncontaminated outside air could remove the need to filterthe air and reduce filtering cost. However, increasing the amount ofoutside air may increase the amount of energy consumed to maintain adesired temperature. Thus, the filtering cost must be compared to theincreased heating or cooling cost to select the most cost effectivesolution. Heretofore, no system has addressed these energy consumptionconcerns.

A need remains within the industry to provide an air conditioning systemwhich overcomes the disadvantages noted above and experiencedheretofore. It is an object of the present invention to provide such asystem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a control system foran air conditioning unit which complies with the IAQ standards bymonitoring return and outside air and selectively drawing optimalpercentages thereof based on the enthalpy and air quality.

It is a further object of the present invention to provide a controlsystem for an air conditioning unit which minimizes energy consumptionand the cost of replacement filters by bypassing the high efficiencyfilters when the air quality permits while continually meeting the IAQstandards.

It is a further object of the invention to provide a control system foran air conditioning unit with a filtration system which may be utilizedto achieve high capacity filtering and which may be bypassed whenminimum or no filtration of the supply air is required so that theincreased operating costs resulting from the filter system are avoided.

It is also an object of this invention to provide an air conditioningunit having a high capacity filtration system with an air qualitysampler so that supply air which requires little or no filtering may beautomatically detected and routed to bypass the filtration system,thereby permitting more economical operation of the system by reducingpressure losses in the system.

It is a further object to provide a controller that monitors theoutside, return, and supply air quality and when the indoor CO₂ levelrises above a prescribed level (e.g., 1000 PPM) and the incrementaldifference between the return air CO₂ level and the outdoor air CO₂level rise above a prescribed level (e.g., 500 PPM), the controllerincreases the outdoor air damper minimum flow to a prescribed occupancyrequirement level.

Finally, it is an object to provide a control module that monitors VOCor other specific contaminant levels in indoor and return air and basedupon these readings, the control module instigates a control algorithmthat compares outdoor and return air enthalpy and VOC levels todetermine whether it is feasible to reduce the amount of outdoor air toa minimum and to modulate the dampers open to admit return air to thegas phase or contaminant filter media or, alternatively, to bypass thefilter media and to open the outdoor air damper further to dilute theindoor air contaminant level. By adding a flow measurement sensingoption, the inventive control system can be expanded to calculate theoptimal sequence based on the above variables plus the filtering cost,versus energy cost of heating or cooling the added air brought in fordilution.

To accomplish these and other objects of the invention, a control systemis provided for an air conditioning unit with a substantially enclosinghousing and inlet and discharge openings in the housing. An internallymounted fan draws outside and return air through the inlet opening toform supply air which passes through a high capacity filtration systemand then through a temperature conditioning system and is subsequentlydischarged through the discharge opening. A bypass passage is providedso that supply air which does not require extensive filtration may berouted around the filtration system. Flow control devices such as airdampers are positioned to block passage of the supply air through thehigh capacity filters and route the supply air through the bypass.Additional dampers are provided proximate the return and outside airinlets to control the percentage of each within the supply air. Thebypass permits the unit to be economically operated without highcapacity filtration during periods when the supply air is of sufficientair quality. When filtration is required, the flow control devices arerepositioned to block the bypass and route the supply air through thefiltration system and to vary the percentages of return and outside air.One or more air quality detectors may be utilized to sample the airquality automatically in order to position the flow control devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numerals are usedto indicate like parts in the various views:

FIG. 1 is a side elevational view of an air conditioning unit of thepresent invention with portions removed to illustrate details ofconstruction and to show a series of flow control devices positioned todirect the supply air through a high capacity filtration system;

FIG. 2 illustrates a block diagram of an alternative embodiment whereina high capacity filtration system, sensors and controller according tothe present invention have been retrofitted to a conventional airconditioning unit;

FIG. 3A illustrates a processing sequence undergone by the presentinvention to control the conditioning unit illustrated in FIG. 1;

FIG. 3B illustrates a processing sequence undergone by the presentsystem during control of the apparatus illustrated in FIG. 1;

FIG. 4A illustrates a processing sequence undergone by a control systemaccording to the present invention to control the apparatus of FIG. 2;

FIG. 4B illustrates a processing sequence undergone by a control systemaccording to the present invention to control the apparatus of FIG. 2;

FIG. 5 illustrates an alternative processing sequence followed during asteady state condition to calculate the equivalency ratio between thepercentage change in outside air and the percentage change in filteredair;

FIG. 6 illustrates a look up table to be utilized to calculate thecooling/heating costs and the filtration costs for a given flow of cfmof outside air; and

FIG. 7 illustrates the sequence followed by the controller to utilizethe lookup table of FIG. 6 to calculate cooling/heating costs andfiltration costs.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings in greater detail, an air conditioningunit is represented broadly in FIG. 1 by the numeral 10. The unit 10 isadapted for coupling with a building air conveyance system to delivertemperature conditioned air to rooms or other areas within the buildingwhich are in need of temperature conditioning.

Unit 10 comprises a substantially enclosing housing 12 formed fromopposed end walls, vertical side walls, and horizontal walls. An outsideair inlet 26 and a return air inlet 28 are positioned in the end walland the horizontal wall, respectively. Flow control dampers 30 and 32are associated with inlets 26 and 28, respectively, to regulate the flowof supply air through the inlets. The flow control dampers 30 and 32 arecontrolled via stepper motors (not shown) which are controlled by acontrol system (explained in more detail in connection with FIGS. 3A and3B).

The air inlets 26 and 28 feed outside and return air into an intakechamber 34 which is defined by the housing walls and a flow controldevice 42. The outside and return air combine to form mixed unprocessedair within the intake chamber 34. A processed chamber 35 is formedadjacent the discharge port downstream of the control device 42 anddownstream of the heating and cooling coils 50 and 51. The processedchamber 35 contains processed supply air. A filter 38 is positionedadjacent the inlets 26 and 28 in the intake chamber and is designed forrough screening and removal of large airborne particles from the supplyair entering the chamber 34. A high capacity filtration system 40 ispositioned downstream from the initial filter 38 and adjacent thecontrol device 42. Many suitable types of mechanical, electronic,adsorptive and other filtration devices (e.g., HEPA filters, carbonfilters and the like) may be utilized alone or in combination in thehigh capacity filtration system 40 and the invention is not limited toany particular type of filter. The high capacity filters may be arrangedparallel to one another, in series or in any combination thereof. Thefilters, however, must provide the necessary filtration capacity toachieve a predetermined air quality under peak load conditions. The highcapacity filtration system 40 shown does not completely span theinterior cross-sectional area of the chamber 34. Instead, one end of thefiltration system 40 forms a passage 48 with the wall of the housing 12between the intake chamber 34 and the filtered chamber 35. As will besubsequently described, that portion of the interior cross-sectionalarea not occupied by the high capacity filtration system 40 is availablefor air flow to bypass the filtration system 40.

Optionally, the filtration system 40 may be configured to span theinterior cross-sectional area of the chamber 34. In this instance, aseparate external bypass passage would be provided having separateductwork accepting air from the intake side of the filtration system 40and returning it to the outlet side thereof. The bypass conduit wouldinclude a damper to close the bypass to admit a desired amount of airabout the filtration system 40.

Optionally, a second outside air inlet 27 may be provided proximate thedownstream side of the high capacity filtration system 40, and theupstream side of the heating and cooling coils 50 and 51. An outside airdamper 31 is controlled by the controller 29 to open and close the inlet27 depending upon the outside air quality. This outside air may or maynot require prefiltering before entering the inlet 27. As a furtheralternative, the outside air inlet 26 may be removed or entirely closedto ensure that only return air passes through the high capacityfiltration system 40 while outside air is admitted at inlet 27.

The flow control device 42 is positioned intermediate the initial filter38 and the high capacity filtration system 40 to direct air flow throughor about the filtration system 40. Flow control device 42 includes anupper section 44 (often referred to as a bypass damper) and anindependently operable lower section 46 (often referred to as a facedamper). The lower section 46 is coextensive with high capacityfiltration system 40 and is operable to direct intake air from theintake chamber 34 and through the filtration system 40. Flow controlupper section 44 directs intake air from the intake chamber 34 through apassage 48 which bypasses the filtration system 40. The flow controldevice 42 may comprise any of many suitable types of devices such asdampers (controlled by a stepper motor or other suitable activator)which may be moved between an open position which permits the passage ofair streams and a closed position which blocks the air stream flow.

The bypass passage 48 is positioned and configured to minimize pressuredrop between the intake and filtered chambers 34 and 35 as air isdiverted around the high capacity filtration system 40. In theillustrated embodiment, the bypass passage is positioned above thefiltration system, but it is to be understood that the bypass passage 48may assume other configurations and locations.

Heating and cooling units 50 and 51 are positioned upstream ordownstream of the high capacity filtration system 40 to provide fortemperature conditioning of the supply air as it passes through theunit. A fan 52 is also positioned internally of housing 12 and ismounted on a platform 54 which is spaced above the horizontal wall bytorsion springs 56. The fan 52 includes a motor 58 coupled thereto by apair of pulleys and a drive belt 66. By way of example only, the fan 52may be a cage type fan having a discharge end mounted to dischargeductwork in the end of the housing. It is to be understood that otherfan configurations may be utilized in place of that illustrated anddescribed. It is also to be understood that the heating and coolingcoils, fan and filters may be arranged in any desired order. The mixedair usually passes through a prefilter before contacting the HEPA orhigh efficiency filter.

The controller 29 may operate the fan 52 as a constant flow system tosupply a constant cubic feet per minute (cfm) of air to the buildingspace at all times. Alternatively, the controller 29 and fan 52 mayoperate as a variable airflow system (VAV) to adjust the total cubicfeet per minute (cfm) of air supplied to the building space through theductwork. If operated as a VAV system, the air flow varies based onsystem requirements as determined by one or more static sensors in thesupply ducts or through monitoring individual zone controllers.Alternatively, additional fans may be used, such as to draw return airout of the building space to an exhaust port outside the building.

Optionally, an air quality detector 74 is positioned internally orexternally of housing 12 and is operably coupled with a controller 29that controls the flow control device 42. Detector 74 may comprise anyof various suitable devices which are capable of continuously orintermittently sampling the supply air in order for a controller 29 todetermine the level of one or more pollutants, the air flow rate and/orthe enthalpy thereof. For example, detectors are available which measurecarbon dioxide levels as an indication of air quality. Other detectorsmay measure the levels of airborne particulate, sulfur dioxide, carbondioxide, carbon monoxide, various VOCs and the like.

Additional sensors 76 and 78 are provided to sense the air quality, airflow rate, and enthalpy for return air and for outside air.Alternatively, separate sensors may be provided for enthalpy and for airquality. Further, sensors 76 and 78 may be located at various pointswithin the air flow system so long as they accurately register theenthalpy and quality of outside and return air. The sensors 76 and 78are connected to the controller 29 and deliver control signals theretorepresentative of the surrounding air's quality and enthalpy. Asexplained below, the controller 29 adjusts the dampers 26 and 28 and theflow control device 42 to optimize the operation of the air conditioningsystem.

An air flow measuring station 53 is provided downstream of the point atwhich the return and outside air combine to form mixed air. Themeasuring station 53 detects the total air flow through the housing 12at an instant in time (i.e., the current total cfm_(MA) of mixed air).This measurement is passed to the controller 29. Optionally, a secondair flow measuring station 55 may be provided to monitor the currenttotal cfm_(OA) of outside air being drawn into the intake chamber 34.This measurement is also passed to the controller 29.

In operation, the fan 52 directs air through the unit 10 from either orboth of the outside air inlet 26 and return air inlet 28. The dampers 30and 32 associated with the respective inlets may be positioned to permitan influx of only recirculated air or outside air or a mixture of both.

Air entering the intake chamber 34 is initially screened by filter 38which removes large airborne particles. The intake air next encountersthe flow control device 42 and, depending upon the positioning of theflow control upper and lower sections 42 and 44, is directed througheither the high capacity filtration system 40 or the bypass passage 48,or both. Air either drawn or blown through the filtration system 40 hasundesired pollutants removed therefrom while the air routed through thebypass passage 48 is untreated. Filtered (or unfiltered depending uponthe system configuration) air is then heated or cooled by heating orcooling unit 50 and 51 and passed into the discharge chamber 72. Theprocessed supply air is then discharged through the outlet (not shown)and conveyed by the building duct work to the areas or rooms to beconditioned.

The flow control device 42 is effective to reduce the operational costsof the unit 10 by permitting a portion or all of the supply air tobypass the high capacity filtration system 40 which would otherwisecause a significant pressure drop in the system. When bypass of thefiltration system is desired, the flow control lower section 46 isclosed and the upper section 44 is opened. Likewise, when filtration ofthe mixed air is desired, the lower section 46 is opened and the uppersection 44 is closed. The upper and lower sections may also be placed inintermediate positions which permit partial flow through both the bypasspassage 48 and filtration system 40. Bypassing the filter whenfiltration is not required extends the filter life and reduces costsassociated with the material and labor of filter replacement.

Automatic positioning of the flow control device 42 in response to airquality conditions is directed by the controller based on the airquality detectors 74, 76 and 78. The detectors 74, 76 and 78 are alsooperably coupled via the controller 29 with the outside and return airinlet dampers 30 and 32 to establish the most energy efficient operatingconfiguration under the current air quality and other operatingconditions. For example, under a minimal thermal load with no heating,cooling or filtration of the outside air required, the outside airdamper 30 and flow control upper section 44 may be opened and the returnair damper 32 and flow control lower section 46 closed. Provided theoutdoor air is of satisfactory quality, acceptable air quality levelsare thereby maintained in the building without the pressure drop andincreased operating costs created by high capacity filtration systems.As another example, when minimum outside air is desired, the return airdamper is opened and the outside air damper is moved to a minimum openposition. The flow control device 42 is then positioned to either routethe return air through the filtration system 40 or through the bypasspassage 48, as dictated by the filtration requirements.

FIGS. 3A and 3B illustrate the processing sequence undergone bycontroller 29 to adjust the dampers 30 and 32 and the flow controldevice 42. Beginning with FIG. 3A, when the controller 29 starts 100, ittests the enthalpy, quality and air flow rates of the outside, return,mixed and supply air. The controller 29 next compares the enthalpyreadings for the outside and return air to determine whether the outsideair enthalpy is greater than the return air enthalpy (step 102). If thisdecision is answered in the negative, the controller 29 next utilizesthe reading obtained within sensor 78 for the quality of the outside airand determines whether this quality falls below a minimum acceptablestandard (step 104). If the quality of the outside air is such that itcontains more contaminants than are acceptable, flow passes to step 120at which the controller 29 positions the outside air damper 30 to aminimum setting. In this manner, the controller 29 sets the damper 30 toadmit the minimum allowed amount of outside air (e.g., 100 cfm). Eachtime the controller 29 reaches a RETURN block it waits a preset periodof time and repeats the processing sequence of FIGS. 3A and 3B.

Returning to step 104, when the outside air quality is identified to beabove the minimum acceptable level, flow passes to step 106 at which thecontroller 29 modulates the outside air damper 30 to effect an"economizer cycle." The "economizer cycle" is described in the "ASHRAEHandbook 1991", Chapter 41, which is incorporated by reference. Ingeneral, during an economizer cycle, the outside air damper 30 isadjusted to admit an amount of outside air sufficient to maintain themixed air temperature at a desired level. When processing flow reachesstep 106, the outdoor air enthalpy must be less than the return airenthalpy. Thus, as the percentage of outside air increases forming themixed air (within the intake chamber 34), the enthalpy of the mixed airdecreases.

The economizer cycle is based on mixed air temperature and, duringoperation, always opens the outside air damper as much as required tomaintain the mixed air (or supply air) temperature at the desiredsetpoint (e.g., 50° F.). If the temperature of the mixed air deviatesbelow this set point, in the economizer cycle, the outdoor air damper ismoved toward the closed position (to admit less cool outside air), andthe return air damper is opened gradually to admit more warm return air)until the mixed air temperature reaches the set point. Similarly, as thetemperature of the mixed air goes above the set point, in the economizercycle, the outdoor air damper is gradually modulated open.

Generally, the heating coil is only required due to an adjustment madeduring the economizer cycle, when the outside air temperature is verylow and the minimum flow rate of outside air is relatively high. Theeconomizer cycle is always required to use the minimum outside air flowrate, and thus in this situation the mixed air temperature would fallbelow the setpoint (e.g., if O.A. temp.=-20° F. and minimum O.A. flow is40% of total air flow and R.A. temp.=80° F., then the mixed air temp.would equal 0.4(-20)+0.6(80)=40°). In this example, the heating coilwould be required to raise the mixed air temperature to 50° F (thesetpoint).

The "economizer cycle" of step 106 is carried out consistent with anyone of several known damper control techniques such as with fuzzy logicor a PID algorithm, such as one disclosed in the "ASHRAE 1991Applications Handbook", Chapter 41, and the "Economizer ControlDescription ASHRAE 1991 Applications Handbook" Chapter 41, both of whichare expressly incorporated herein by reference. The modulation functionvaries the outside air damper setting based on the supply air enthalpyset point and the error from set point as sensed by a temperature sensorapproximately located.

Next, control passes to step 108 at which the controller 29 determineswhether the quality of the return air falls below the specificationlimit. If the quality of the return air is acceptable, the return airneed not be filtered by the high capacity filtration system 40. Thus,control passes to step 118, at which the controller 29 directs thecontrol device 42 to close the lower or face damper section 46 and toopen the upper or bypass damper section 44. Returning to step 108, ifthe return air is unacceptable, such air must be filtered, if reused, bythe high capacity filtration system 40.

However, the outside air quality has been identified as being above theacceptable standard (step 104) and need not be filtered. Thus, it may bepreferable to utilize clean cool outside air, rather than filteredreturn air to improve the mixed air quality. Along this line, in step109, the controller obtains a new damper position for the outside airdamper which represents a "preferred" or percentage change (%ΔOA) in theamount of air admitted through the damper 30 (e.g., 10% more outside airwhen the return air is slightly dirty, 20% more outside air when thereturn air is very dirty and the like). The "preferred change may alsobe represented as a change in cubic feet per minute of outside air flowΔcfm_(OA) by multiplying the percentage change %ΔOA by the current airflow volume of outside air cfm_(COA). The damper position obtained instep 109 represents a damper change above and beyond any adjustmenteffected in the economizer cycle within step 106. The damper positionobtained in step 109 further represents a "preferred" setting since thecontroller 29 does not automatically adjust the damper 30 to thissetting.

Prior to rendering the preferred setting adjustment, the controller 29must first determine whether the conditioning unit is capable ofprocessing (i.e., heating) the amount of outside air that will beadmitted through the damper 30 if it is adjusted by the percentagechange (%ΔOA) equal to the "preferred" setting. Some amount of heatingwill be required since the economizer cycle in step 106 has alreadyadmitted the maximum amount of outside air that is acceptable whilemaintaining the mixed air temperature at the setpoint without using theheating coil. The controller also obtains new readings from the sensors,at least for outside, return and mixed air enthalpy and air flow toaccount for any adjustments effected in step 106.

After step 109, control passes to step 110, at which the controllerdetermines whether the heating coil is capable of utilizing the amountof additional outside air that will be introduced if the damper 30 isadjusted by the percentage change (%ΔOA). The controller uses thefollowing equation to calculate the total amount of outside air thatwill be used if moved to the "preferred" setting: cfm_(POA)=(1+%ΔOA)(cfm_(COA)); wherein (%ΔOA) represents the percentage change inoutside air obtained in step 109, and cfm_(COA) represents the totalcurrent outside air flow admitted through the outside air damper 30.

The current outside air flow cfm_(COA) may be sensed directly by the airflow sensor 55 or 78. Alternatively, the total current outside air flowcfm_(COA) may be obtained by comparing the pressure drop across theoutside air damper 30 and the position of the outside air damper bladeswith a calibration curve for that damper. As a further alternative, theoutside air flow cfm_(COA) may be calculated based upon the temperatureof the outside and return air (t_(OA) and t_(RA)) and the total currentmixed air flow cfm_(COA), provided that the outside air temperature issomewhat different from the return air temperature. For example, usingthis final method, the outside air flow cfm_(COA) may be calculatedbased on the equation: (cfm_(COA))(t_(OA))+(cfm_(MA)-cfm_(COA))(t_(RA))=(cfm_(MA))(t_(MA)); where t_(MA) represents themeasured temperature of the mixed air within intake chamber 34 and theremaining variables represent the air flow rates and temperature asexplained above. The variables t_(OA), t_(RA), cfm_(MA), and t_(MA) maybe obtained from the appropriate sensors.

For instance, if it is assumed that the outside and return airtemperatures are 40° F. and 80° F., respectively, the set pointtemperature for the mixed air is 50° F., and the mixed air flow rate is8000 cfm, then the maximum outside air flow rate cfm_(MOA) equals 6000cfm or 75% of the total mixed air. In other words, the mixed air mayinclude 6000 cfm or 75% outside air without turning ON the heating coilto maintain the mixed air temperature at 50° F. If the amount of outsideair increases above 75%, then the heating coil must be activated tomaintain the supply air temperature at the set point.

If it is assumed that the damper 30 is currently set at 75% open withinthe economizer cycle (i.e., cfm_(COA) =6000 cfm when the mixed air flowequals 8000 cfm) and that the preferred percentage change %ΔOA equals20%, then the damper 30 will be preferably adjusted to admit 7200 cfm ofoutside air (i.e., cfm_(POA=)(1+%ΔOA) (cfm_(COA)).

The preferred outside air flow rate cfm_(POA), and the outside andreturn air temperatures t_(OA) and t_(RA) are used to calculate theresulting mixed air temperature t_(MA-RES) that will result when thepreferred outside air flow rate cfm_(POA) is used (i.e.,t_(MA-RES=)[(t_(OA)) (cfm_(POA))]/[cfm_(MA]+)[(t_(RA)) (cfm_(MA)-cfm_(POA))]/[cfm_(MA) ]. In the foregoing example, t_(MA-RES) =44°F.=[(40) (7200)]/[8000]+80[8000-7200)]/8000. Once the resulting mixedair temperature is calculated, the heat energy may be calculated inBTUs/HR that will be used to maintain the supply air at the desiredtemperature t_(SA) based on the equation:

BTU/HR=cfm_(MA) (K)(t_(MA-SET) -t_(MA-RES)); where K represents aconstant (generally 1.085 for standard air density i.e., sea level at70° F.). If the altitude or temperature are not at standard values, theconstant must be corrected accordingly. cfm_(MA) represents the totalair flow, t_(MA-RES) represents the resulting mixed air temperaturecalculated above and t_(MA-SET) represents the mixed or supply airdesired setpoint temperature. If cfm_(MA) equals 8000 cfm, the supplyair temperature equals 50 and the resulting mix air temperature equals44, the foregoing equation may be solved as follows:

    8000(1.085)(50-44)=52080 BTU per hour.

The heating coil capacity (as specified and installed for the HVACsystem requirements) is sufficient to heat the 1200 cfm of additionaloutside air. Thus, the capacity of the heating coil is not exceededwithin step 110 and flow passes to step 111. Alternatively, if theheating capacity of the coil was exceeded by the additional increment ofoutside air, control would pass to point 116 wherein flow moves to FIG.3B.

Also in step 109, the controller obtains a preferable filter changeΔcfm_(FLTR) (and percentage filter change %Δcfm_(FLTR)) to adjust theface damper 46 to allow additional return air through the high capacityfiltration system 40. This preferred filter change Δcfm_(FLTR) may be apreset increment, such as 20% or 0.2(cfm_(FLTR)) wherein the face damper46 will preferably be opened to admit an additional 20% more air throughthe filter 40. Again the filter damper change Δcfm_(FLTR) is a"preferred" setting since it is not yet implemented. First, thecontroller must determine whether it is more feasible to open the damper46 by this additional amount Δcfm_(FLTR), or alternatively to open theoutside air damper 30 to admit additional outside air Δcfm_(OA).

Optionally, the filter damper change Δcfm_(FLTR) may be obtained from aproportionality table that establishes a proportional relationshipbetween the filter damper change Δcfm_(FLTR) and the outside air damperchange Δcfm_(OA). By way of example, if the outside air damper changeΔcfm_(OA) is calculated to equal 10%, the table would contain anequivalent increase in the filter damper change Δcfm_(FLTR) to filter anequivalent incremental increase in return or mixed air (e.g. 5%, 20% andthe like depending upon the quality of the return air).

Next, control passes to step 112, at which the controller 29 is nowpresented with two manners to achieve the desired supply air quality andtemperature, namely (1) through maximizing the use of outside air or (2)through maximizing the use of return air. Maximizing the use of outsideair requires the system to incur additional costs associated withheating this air, while maximizing the use of return air requires thesystem to incur costs associated with filtering the return air. Thus,the controller 29 must determine (at step 112) the most economical wayof achieving the desired supply air quality and temperature. At step112, the controller compares the additional heating costs with theadditional filtering costs. If the former option is more feasible,control passes to point 116. If the latter option is more feasible,control passes to point 114.

In step 112, the foregoing decision is determined based upon thefollowing equation:

    Δ$.sub.HEAT <Δ$.sub.FLTR ;

where Δ$_(HEAT) represents the cost of the energy required to heat themixed air from its resulting air temperature t_(MA-RES) to the setpointt_(SA-SET) once the preferred change in outside air %ΔOA is added andΔ$_(FLTR) represents the cost of the energy required to filter anequivalent amount of return air Δcfm_(FLTR). The cost of the heat energyis obtained based on the following equation:

    Δ$.sub.HEAT =(cfm.sub.MA)(K)(t.sub.MA-SET -t.sub.MA-RES)($/BTU);

wherein $/BTU represents the cost per BTU of energy and the variablescfm_(MA), K, t_(MA-SET) and t_(MA-RES) are explained above. The cost perBTU of energy may be varied depending upon the time of day to accountfor peak-time and non-peak-time prices charged by the power company.

The increase in the cost of the filter energy Δ$_(FLTR), which would berequired if the filter damper 46 were opened by the amount Δcfm_(FLTR)to filter additional mixed or return air, equals the sum of the fanenergy cost Δ$_(FAN) and the filter replacement/use costΔ$_(REPLACEMENT) (i.e., Δ$_(FLTR) =Δ$_(FAN) +Δ$_(REPLACEMENT)). Thetotal filtering cost includes the cost of the increased fan energy andthe incremental filter material and replacement costs. The filterreplacement cost Δ$_(REPLACEMENT) may be obtained from the manufacturerand may be based on a cost per cfm/HR passed through the filter (i.e.,Δ$_(REPLACEMENT) =(replacement cost per cfm)(cfm_(MA)); where cfm_(MA)represents the air flow through the filter after the face damper isopened %Δcfm_(FLTR)).

The additional cost for fan energy Δ$_(FAN) results from the increase instatic pressure drop within the conditioning system. The change instatic pressure drop equals a change in static pressure drop across thehigh efficiency and/or gas phase filters Δ_(DP) _(FL) caused by theincreased air flow which will result when the face damper is opened%Δcfm_(FLTR). The cost of the increase in fan energy (Δ$_(FAN)) maybecalculated based on the equation:

    Δ$.sub.FAN =[E.sub.CFAN (DP.sub.FN2 /DP.sub.FN1)]-E.sub.CFAN ;

wherein E_(CFAN) represents the energy currently being consumed by thefan, DP_(FN1) represents the current static pressure drop across the fan(before the face damper is moved) and DP_(FN2) represents the futurestatic pressure drop across the fan (after the face damper is moved).The future static pressure drop DP_(FN2) maybe calculated as follows:

    DP.sub.FN2 =DP.sub.FN1 +DP.sub.FL2 -DP.sub.FL1 ;

wherein DP_(FL1) and DP_(FL2) represent the current static pressure dropacross the high capacity filtering system 40 (before the face damper ismoved) and the future static pressure drop across the filtering system40 (after the face damper is moved). The DP_(FL2) -DP_(FL1) represents adifferential increase in filter drop. The future static pressure dropacross the filter DP_(FL2) maybe calculated according to the followingequation:

    DP.sub.FL2 =DP.sub.FL1 (1+%ΔFLTR.sub.CHANGE).sup.2 ;

wherein %ΔFLTR_(CHANGE) represents a percentage change in the air flowthrough the filtering system 40 when the face damper 46 is moved by thepreferred amount %Δcfm_(FLTR). The percentage change %ΔFLTR_(CHANGE) isobtained in step 111 and represents the ratio(%Δcfm_(FLTR))/(%cfm_(CURRENT)); wherein %cfm_(FLTR) represents thepreferred percentage change in air flow through the filter damper 46 and%cfm_(CURRENT) represents the percentage of outside air passing throughthe filter before the damper 46 is adjusted to admit the additionalamount %Δcfm_(FLTR).

During an initial pass through step 111, in which the face damper 46 iscompletely closed, the controller 29 determines that the face damper 46will be opened by a preset amount (e.g., 20%) which corresponds to apreset pressure drop (e.g., 0.08 inches). Thus, on the first passthrough step 111 when the face damper 46 is closed (but will bepreferably opened 20%) , the current static pressure drop DP_(FL1) =0and the future static pressure drop DP_(FL2) is automatically set toequal 0.8 inches. In this case, the differential pressure drop DP_(FL2)-DP_(FL1) =0.08 inches. On the second pass through step 111, when theface damper 46 is already open 20% (and will preferably opened anadditional 20%), the current static pressure drop DP_(FL1) =0.08 inchesand the future static pressure drop DP_(FL2) =0.32 inches (i.e.,DP_(FL2) =DP_(FL1) (1+(%Δcfm_(FLTR))/(%cfm_(CURRENT)))² or DP_(FL2)=0.08 {1+(0.2/0.2)}²). The differential pressure drop DP_(FL2) -DP_(FL1)=0.24 (i.e., 0.32-0.08).

On the third pass through step 111, when the face damper 46 is alreadyopen 40% (and will preferably opened an additional 20%), the currentstatic pressure drop DP_(FL1) =0.32 inches and the future staticpressure drop DP_(FL2) =0.72 inches (i.e., DP_(FL2) =DP_(FL1)(1+(%Δcfm_(FLTR))/(%cfm_(CURRENT)))² or DP_(FL) ₂ =0.32 {1+(0.2/0.4)}²).The differential pressure drop DP_(FL2) -DP_(FL1) =0.40 (i.e.,0.72-0.32).

On the fourth pass through step 111, when the face damper 46 is alreadyopen 60% (and will preferably opened an additional 20%), the currentstatic pressure drop DP_(FL1) =0.72 inches and the future staticpressure drop DP_(FL2) =1.28 inches (i.e., DP_(FL2) =DP_(FL1)(1+(%Δcfm_(FLTR))/(%cfm_(CURRENT)))² or DP_(FL2) =0.72 {1+(0.2/0.6)}²) .The differential pressure drop DP_(FL2) -DP_(FL1) =0.559 (i.e.,1.28-0.72).

On the fifth pass through step 111, when the face damper 46 is alreadyopen 80% (and will preferably opened an additional 20%), the currentstatic pressure drop DP_(FL1) =1.24 inches and the future staticpressure drop DP_(FL) ₂ =2.0 inches (i.e., DP_(FL2) =DP_(FL1)(1+(%Δcfm_(FLTR))/(%cfm_(CURRENT)))₂ or DP_(FL2) =1.24 {1+(0.2/0.8)}²).The differential pressure drop DP_(FL2) -DP_(FL1) =0.76 (i.e.,2.0-1.24).

In addition, the fan energy equation may account for motor and fanefficiencies. The cost per kilowatt of fan energy may also be variedwith different times of day to account for variations in the charges ofpower companies as peak times of the day. The filter costs may accountfor the cost of the filter, replacement costs, and the like. The energydrawn by the fan $_(FAN) may be obtained by monitoring the powerconsumption of the fan (i.e., by an amp meter or a watt meter) or bymonitoring the output of the motor speed controller which dictates therotational speed of the fan and may have logic to output a signalproportional to total power draw. The static pressure drop DP_(FN1)across the fan may be measured, such as by a transducer and the like.Optionally, the percent increase in static pressure drop across thefilter (ΔDP_(FN)) occurring when additional air is passed through thehigh efficiency filter may be obtained by directly measuring the staticpressure drop across the filter or by utilizing a look up table which isadjusted to account for "loading" of the filter occurring over time(i.e., an increase in static pressure drop for a given air flow rate dueto clogging of the filter).

As illustrated in FIG. 3B, if the heating energy cost Δ$_(HEAT) is lessthan the filtering energy cost Δ$_(FLTR) the controller obtains amodulation amount for the outside air damper (Δcfm_(OA)) based on thereturn air quality (step 145). Next, the controller adjusts the outsideair damper 30 (step 146) by opening it an amount equal to the outsideair damper change Δcfm_(OA). If the heating energy cost Δ$_(HEAT) isgreater than the filtering cost Δ$_(FLTR) the controller allows theoutside air damper 30 to modulate on the normal economizer cycle (step106). Next, the controller obtains a modulation amount for the facedamper (Δcfm_(FLTR)) based on the return air quality and moves the facedamper accordingly (step 141).

Returning to FIG. 3A, at step 102, when the enthalpy of the outside airis determined to be greater than the enthalpy of the return air, controlpasses to step 122. At step 122, the controller tests the outside airquality to determine whether it meets the minimum acceptable standard.If this standard is not met, control passes to step 120 at which theoutside air damper 30 is set to admit a minimum acceptable amount ofoutside air. At step 122, if the outside air quality is above theacceptable standard, control passes to step 124 at which the controller29 determines whether the return air quality is below the minimumstandard. If not, the flow control device 40 is adjusted to close theface damper 46 and open the upper section damper 42 in order to bypassthe high capacity filtration system 40. Thereafter, the outside airdamper is set to admit the minimum amount of outside air (step 128).

At step 124, if the quality of the return air is below the minimumacceptable level, control passes to step 130 at which the controllerdetermines whether the cooling coil capacity is capable of reducing theenthalpy of the mixed air to a desired enthalpy set point (e.g., 50° F.at 98% humidity) when the amount of outside air is increased by thepercentage change %Δcfm_(OA). The cooling requirement is calculatedbased on equations similar to that used above to calculate the resultingmixed air temperature and the heating requirement, except that thetemperature variables t_(MA-SET) and t_(MA-RES) for the mixed airsetpoint and the resulting mixed air temperature are replaced withenthalpy variables for the mixed air enthalpy set point h_(MA-SET) andthe resulting mixed air enthalpy h_(MA-RES). The resulting mixed airenthalpy h_(MA-RES) is calculated based on the outside and return airenthaply measurements and based upon the percentage of outside andreturn air being added to the mixed air. This calculation may beeffected in a variety of known manners once the enthalpy and air flowmeasurements are taken for the outside and return air.

Once the resulting mixed air temperature is calculated, the coolingenergy may be calculated in BTUs/HR that will be used to maintain themixed air at the desired enthalpy setpoint h_(MA-SET) based on theequation:

    BTU/HR=cfm.sub.MA (K) (h.sub.MA-SET -h.sub.MA-RES);

where K represents a constant (generally 4.5 at standard air density) toconvert to BTUs per HR, cfm_(MA) represents the total air flow,h_(MA-RES) represents the resulting mixed air enthalpy calculated aboveand h_(MA-SET) represents the mixed or supply air desired setpointenthalpy. If the altitude or temperature are not at standard values, theconstant must be corrected accordingly. The preferred amount of outsideair cfmp0_(A) is obtained, prior to step 130, along with the maximumuncooled cfm of outside air cfm_(MOA) that may be introduced withoutusing the cooling coil to maintain the mixed air enthalpy.

By way of example, if the outside air is at 95° F. and 40% humidity andthe return air is at 78° F. and 45% humidity, the mixed air will be 82°F. at 45% humidity. If the supply air is preferably maintained at 55° F.and 98% humidity, the cooling coil will be required to reduce theenthalpy of the mixed air by Δh. Once this enthalpy change Δh iscalculated, it is converted to BTUs/hr and compared to the designcooling coil capacity. If the capacity of the cooling coil is notexceeded, control passes to point 134 in FIG. 3B. Otherwise, controlpasses to step 132 at which the outside air damper is positioned toadmit the minimum acceptable amount of outside air. Thereafter, controlpasses to point 116 in FIG. 3B.

It should be noted from the above calculations that, for a givenincremental flow change through the filter, that the differentialpressure (and thus the fan energy) does not change at the same rate. Forexample, during the second pass (or second incremental flow change) thepressure drop equals 0.24 inches. In addition, for the same incrementalchange in cfm through the filter, the pressure drop changes by 0.76inches. Thus, for a given outside air enthalpy level, it may be morecost effective to filter an increment of return air. However, as moreair is passed through the filter, it may reach a balance point where itwould be more economical to use outside air for the next increment of"effective dilution".

The controller reviews this calculation for each process cycle and, ifthe outside air enthalpy changes, or the effective dilution rate foroutside air changes (such as when the outside air becomes morecontaminated) the controller recalculates the economics of filtrationversus dilution and positions the damper to obtain the desired ratio ofoutside air and filtered air.

Returning to FIG. 3B, when flow passes to point 134, the controller 29determines in step 138 the most cost efficient method of reducingcontaminant concentration within the supply air. In particular, thecontroller 29 determines whether the energy cost required to cool theoutside air Δ$_(COOL) exceeds the cost required to filter return airΔ$_(FLTR).

The cooling energy is calculated by the following equation:

    Δ$.sub.COOL =(cfm.sub.MA)(Δh)(K) (cost/BTU of cooling);

where Δh represents the enthalpy differential between the resultingmixed air enthalpy (after introducing the preferred increase in outsideair) and the setpoint mixed air enthalpy, cfm_(MA) represents the mixedair flow rate of the outside air to be cooled and K represents aconstant (4.5) to account for standard air density. The filter cost(which equals energy plus replacement) is calculated as explained abovein connection with step 112.

If the cooling cost Δ$_(COOL) exceeds the filtering cost, control passesto step 140 at which the controller 29 directs the outside air damper 30to be set to its minimum acceptable level. Thereafter, the controller 29modulates the flow control device 40 as explained above (at step 141)and processing flow returns to the starting point 100. If the coolingcost does not exceed the filter cost, flow passes to step 145 at whichthe controller obtains the modulation amount to move the outside airdamper dependent upon the return air quality. Thereafter, the outsideair damper is moved based upon this modulation amount (step 146). Next,the controller determines whether the outside air damper has been opened100% in response to a modulation amount obtained in step 145 based uponthe return air quality (step 147). If not, the system returns to thestarting point. If the outside air damper has been opened 100% inresponse to a poor return air quality signal, flow passes to step 144 atwhich the controller determines whether to adjust the face damper uponthe filter to further filter the mixed air. Flow only passes from step147 to step 141 when the return air quality is extremely poor and theoutside air damper has been opened 100%.

The controller 29 undergoes a second processing sequence (beginning atstep 150) wherein it compares the CO₂ level within the return air to aset limit (e.g., 1000 PPM) (step 151). If that level is exceeded, flowpasses to step 148. If the limit is not exceeded, it compares the returnair CO₂ level with the CO₂ level within the outside air (step 152). Ifthe return air CO₂ level exceeds the outside air CO₂ level by a presetvalue (e.g., 500 PPM) (step 152) flow passes to step 148 at which theoutside air damper 30 is modulated based on the detected CO₂ level inthe return air. If the CO₂ level within the return air does not exceedthat of the outside air by this preset value, flow passes to the RETURNblock. The controller 29 performs the CO₂ evaluation (step 152) inparallel, or in series with the remaining process illustrated in FIGS.3A and 3B.

FIG. 2 illustrates an alternative embodiment for a control system whichmay be added to an existing conditioning system. The system of FIG. 2illustrates a basic air conditioning system above the dashed line 200(generally designated by the reference numeral 202). The system belowthe dash line 200 (generally designated by the reference numeral 204) isreferred to as a pony pack 204. The conditioning unit 202 includes anintake chamber 206 defined by top, bottom and side walls 208, anintermediate end partition 210, an outside air inlet port 212 and areturn air inlet port 214. The inlet ports 212 and 214 are separatelyand controllably closed via dampers 216 and 218, respectively. Thedampers 216 and 218 are controlled via motors 217 and 219, which are inturn controlled via the controller 229.

The intake chamber 206 includes a filter 220 interposed thereacrosswhich represents a normal filter to remove medium and large sizeparticulate material from the supply air. The downstream side of thefilter 220 passes the supply air to heating and cooling coils 222 and224. Air discharged from the heating and cooling coils 222 and 224passes to the discharge fan 226 which directs the discharge air throughthe duct work within the building.

The pony pack 204 is added to an existing air conditioning unit toincrease the air filtration capacity and/or to maintain the minimumventilation rate at a desired level and to provide additional cooling orheating capacity. This additional cooling/heating capacity may be neededto handle an increased ventilation air flow volume, such as to achievedilution of contaminates within the buildings space or to comply withfederal, state and local codes. Depending upon the application, the ponypack can be operated continuously whenever the main air condition unitssupply fan 226 operates, or alternatively the pony pack can run in acycled manner as required to maintain controlled minimum ventilation orincreased filtration.

The pony pack 204 is located proximate the intake chamber 206 and isconstructed with top, bottom and side walls 230. The pony pack 204includes a intake chamber 232 defined by the top, bottom and side walls230, an intermediate end partition 234, an outside air inlet port 236and a return air inlet port 238. The return air inlet port 238communicates with the return air inlet port 214 via duct work 240. Theinlet ports 236 and 238 are controllably shut via dampers 242 and 244,respectively, which are controlled via motors 243 and 245. The motors243 and 245 are controlled by the controller 229.

Within the intake chamber 232, a prefilter 240 is optionally providedthereacross to filter medium and larger size particulate material fromthe supply air within the intake chamber 232. Downstream of theprefilter 240 is provided a high efficiency filtration system 242 whichmay include a high efficiency particulate filter (e.g. a HEPA filter)and/or a gas phase filter (e.g. a carbon filter). The control of thepony pack dampers and fan is dependent upon the quality of the outsideand return air. For instance, if the return air quality is identified ashaving high VOC levels, the controller 229 determines if it is moreeconomical to introduce more air through the outside air damper 216 orto run the pony pack fan and introduce outside air through the dampers236 and a portion of the return air through the damper 244 to provideadditional filtering through the high efficiency filter 242. Thecontroller monitors outside, return and filtered air quality for CO₂levels, VOCs and other specified contaminates. In addition, the outsideand return air enthalpy and pressure drop across the high efficiencyfilter is measured. The amount of total system air flow, total air flowthrough the pony pack and outside air flow through the pony pack is alsomeasured. The outside air flow through the pony pack is measured with aconventional air flow station or by measuring the pressure drop acrossthe damper when the damper is in a known position. Alternatively, theforegoing air flow measurements may be obtained through any otheracceptable method of determining differential pressure measurementsbetween two points within an air conditioning system.

Downstream of the high efficiency filtration system 242, a preheat coil244 and cooling coil 246 are provided to adjust the temperature of thesupply air as necessary. The high efficiency filtration system 242 maybe constructed similar to the filtration system 40 in FIG. 1 to includea bypass about the end thereof. A damper system may be provided on theupstream side of the high efficiency or gas phase filter 242 toselectively bypass each filter depending upon quality of the supply air.Downstream of the cooling coil 246, an air flow station 248 is provided.Air passing from the air flow station 248 is delivered to the dischargefan 250 which directs the discharge air into a discharge portion 252which communicates with the chamber 206 proximate the downstream side offilter 220. The air discharged from the fan 250 is combined with thesupply air within the chamber 206 and ultimately delivered via the fan226 to the duct work throughout the building.

The controller 229 monitors sensors 254-257 to identify the enthalpy,air flow rate, and air quality at the position of each sensor. A sensor254 is located proximate the outside air inlet 236 for the pony pack204. A second sensor 255 is located proximate the return air inlet 214.A third sensor 256 is located within the supply chamber 232. A fourthsensor 257 is located within the supply chamber 206. The sensors 254-257may be formed of conventionally known sensor types to monitor enthalpy,air flow rates, and air quality. Optionally, separate sensors may beprovided to monitor each desired characteristic. Optionally, anadditional pressure sensor 258 may be located proximate the filtrationsystem 242 in order to monitor the pressure differential between theintake and outlet sides of the filtration system 242. The pressuresensor 258 may be formed of a differential pressure transducer withpressure sensing tips on each side of the filtration system 242.

Turning to FIGS. 4A and 4B, the processing flow of the controller 229 isexplained hereafter. Beginning with FIG. 4A, initially the controller229 obtains air quality, flow rate and enthalpy measurements for the airin step 500. Next, it compares the outside air enthalpy with the returnair enthalpy (step 502). If the enthalpy of the outside air is less thanthat of the return air, the controller 229 next compares the quality ofthe outside air with the minimum acceptable standard (step 504). If thisquality falls below the minimum standard, the controller 229 determinesthat it is undesirable to utilize outside air and thus drives the motors217, 243 and 245 to move the dampers 244 and 216 toward a closedposition to admit a minimal amount of outside air (step 520). Air flowsensor 248 monitors this airflow and adjusts the speed (or volumecontrol) damper on the fan 250 to assure sufficient air flow. At step504, if the outside air quality is above the minimum acceptablestandard, the controller determines that it is desirable to maximize theuse of outside air and thus modulates (at step 506) the outside airdampers 216 and 242 and return air dampers 218 and 244 to maintain thesupply temperature at a desired level. As explained above in connectionwith FIG. 3A, this modulation is controlled according to a conventionaldamper control routine, such as a fuzzy logic or PID algorithm.

The foregoing modulation is utilized to set the outside air dampers 216and return air damper 218. Next, control passes to step 508 at which thereturn air quality is tested to determine whether it falls below theminimum standard. If not, the controller 229 determines that the supplyair includes an air quality above the minimum standard and thus thecontroller 229 shuts off the discharge fan 250 within the pony pack(step 518) and closes dampers 242 and 244. At this point, the controller229 has effectively removed the pony pack from the processing flow sincethe high efficiency filters 242 and 246 therein are not needed. Byremoving the pony pack 204 from the conditioning loop, the controller229 improves the conditioning system's energy efficiency and lengthensthe life of the high efficiency filter 240.

Returning to step 508, when the return air quality is determined to bebillow the minimum acceptable standard, the controller 229 determinesthat it may be preferable to maximize the use of outside air (which hasbeen identified in step 504 to have an air quality above the minimumstandard) and to minimize the use of return air. As explained above,outside air can only be maximized to the extent that the heating andcooling coils can maintain the supply air's desired temperature. Thus,at step 509, it obtains a preferred change in outside air flowΔcfm_(OA). Next, the controller 229 determines whether the heating coil222 has a capacity capable of heating the air within the chamber 206 tothe desired supply air temperature if an increased amount Δcfm_(OA) ofoutside air is utilized (step 510). The heating capacity equals that ofthe heating coil 222 since the heating coil 222 will perform all heatingof the mixed air within chamber 206. The heating differential cfm_(HEAT)is based upon the same equation as utilized in FIG. 3A (step 110), inconnection with the first embodiment. If the capacity of the heatingcoil 222 will be exceeded when maximizing usage of outside air, controlpasses to point 516 (within FIG. 4B). If the heating coil capacity isnot exceeded, the controller 229 next determines whether it is more costeffective (at step 512) to maximize outside air usage or to utilize thepony pack to filter contaminants from the return air.

The controller 229 calculates the operating cost of the pony pack whichwould be required to filter the return air sufficiently to lower thecontaminate level to fall within the acceptable standard. The operatingcost of the pony pack is calculated by measuring the actual operatingpower consumption of the fan 250 E_(CFAN) of the pony pack andcalculating the increased power cost Δ$_(PP) required to filter returnair cfm_(PP1) and for filter replacement cost. This calculation of theincreased cost Δ$_(PP) of the pony pack is based on the same equation asused above to calculate the power consumption and filter costs in step112 (FIG. 3A) of the first embodiment, except that this calculation isbased only upon the pony pack fan's power consumption cost $_(PP1) andpony pack filter's pressure drop DP_(FL1). The energy consumed by thefan 226 is not included in this calculation since this element is usedregardless of additional outside air is added or additional return airis filtered. Within step 512, cfm_(PP1) represents the air flow throughthe pony pack at the time the calculation is made to test for energycomparison. cfm_(PP2) represents cfm_(PP1) +Δcfm_(FLTR) ; whereinΔcfm_(FLTR) represents the incremental increase in return air flowthrough the pony pack. If the pony pack is turned OFF at the time of thecalculation within step 512, the controller 229 utilizes fixed initialreference values for cfm_(PP1) and DP_(FL1) (DP_(FL1) represents thestatic pressure drop across the high capacity filter).

Once the heating cost Δ$_(HEAT) and pony pack costs Δ$_(PP) arecalculated, these values are compared in step 512 to determine which ismore efficient. Based upon this comparison, the pony pack is utilized tofilter return air or to utilize outside air introduced through damper216 to dilute existing pollutants within passing the air through thefilters of the pony pack.

Returning to step 502, if the enthalpy of the outside air is determinedto be above that of the return air, control passes to step 522 at whichit is determined whether the cooling coil 224 is capable of cooling themixed air cfm_(COOL) within chamber 206 to the desired enthalpy Δh ifmore than the minimum outdoor is introduced. If the cooling coil'scapacity is exceeded, the controller 229 positions the output air damper216 to admit a minimum amount of outside air (step 520) or if the ponypack is to be used for minimum ventilation control, 216 is closed and242 is opened to a minimum position and the pony pack is started. If thecooling coil capacity is not exceeded, the controller 229 nextdetermines whether return air quality falls below the minimum acceptablelevel (step 524). If not, the controller positions the outside airdamper 216 to admit a minimum amount of air (step 526). Thereafter, thecontroller 229 shuts off the pony pack 204 by closing the dampers 242and 244 and by turning off the fan 250 (step 528). An alternativecontrol scheme could be to close the damper 216 and open damper 236 toadmit a controlled minimum amount of outside air. At step 524, if thereturn air quality is determined to fall below the specificationstandard, the controller 229 next determines whether the outside airquality is above the minimum acceptable standard (step 530). If theoutside air quality is above this standard, control passes to point 534within FIG. 4B. Otherwise, control passes to steps 532 and 533.

Optionally, at step 524, the controller may close the damper 216, opendamper 236 to a minimum and close the damper 244. This ensures that theminimum of cfm of outside air is passed through the system since thepony pack dampers are more accurate to maintain minimum flow control(this alternative flow is illustrated in FIG. 4A via the dashed line527).

Optionally, dampers 218 and 216 operated "together", but in oppositedirections. Thus, when the damper 216 moves toward the open position,the damper 218 is moved an equal amount toward the closed position.

Turning to FIG. 4B, when control enters at point 534, the controllerdetermines (step 538) whether the cooling costs Δ$_(COOL) required tocool the supply air within chamber 206 to the desired level (whenoutside air is increased incrementally) exceeds the costs Δ$_(PP) ofoperating the pony pack to remove contaminants from the return air (whenoutside air is minimized). If the cooling cost exceeds the cost ofoperating the pony pack, control passes to step 544 at which the outsideair damper 216 is closed and the damper 242 is set to admit the minimumlevel of outside air. Thereafter, the pony pack dampers 242 and 244 areset such that the pony pack fan operates to deliver desired air flow(normally maximum design flow) through the filter 240 and processes theminimum amount of outdoor air with the balance made up of return air(step 546). Returning to step 538, if the cooling costs Δ$_(COOL) isbelow the costs of operating the pony pack Δ$_(PP), flow passes to step540 at which the outside air damper 216 is modulated based on a PIDalgorithm (such as explained above). The damper 216 is modulated to itsoptimal setting based upon this algorithm. Thereafter, the controllerdetermines whether the outside air damper is set to a 100% open position(step 542). If the outside air damper 216 is 100% open, the damper 242is opened and the damper 244 is closed and the pony pack fan is run atdesired air flow (step 543). Otherwise, control returns to the startingpoint 500.

As in the first embodiment, the controller 229 performs a separateprocessing sequence to control the damper settings based upon the CO₂levels within the return air and within the outside air. In particular,with reference to FIG. 4B, the controller compares the return air CO₂level to that of the outside air (step 536). If the return air CO₂ leveldoes not exceed a set level (e.g., 1000 ppm) and does not exceed that ofthe outside air by a previously specified amount (e.g., 500 ppm), thecontroller merely returns to the starting point 500. Otherwise, thecontroller moves to step 540 to modulate the output air damper 216according to the PID algorithm. Thereafter, step 542 is repeated asexplained above. The controller 229 may be arranged to perform the CO₂testing prior to or subsequent to the processing flow of FIG. 4A.Alternatively, the controller 229 may represent a multiprocessor and beconfigured to perform the CO₂ testing in parallel with the processingsequence of FIG. 4A.

As is clear from the foregoing detailed explanation, the inventivecontrol system minimizes energy consumption of the conditioning systemby adjusting damper settings based upon outside and return air enthalpy,contaminants (e.g., VOC levels) within the outside and return air, andupon CO₂ levels therein. The controller determines whether it isfeasible to reduce the amount of outside air to a minimum and insteadmodulate the dampers to direct return air through the gas phase andcontaminant filter media or, alternatively, to open the outside airdamper to admit additional outside air in order to dilute the indoor airconcentration of contaminants.

FIG. 5 illustrates a processing sequence which may be followed by thecontroller 29 to obtain the percent change %ΔOA in the outside airdamper and the percent change %Δcfm_(FLTR) in the filter damperposition. The process of FIG. 5 may be used when the system operatesunder steady state conditions. As explained above, these percentagechanges %ΔOA and %Δcfm_(FLTR) may be independently obtained by a PIDalgorithm, fuzzy logic and the like. Alternatively, these percentagesmay be set to preset values, or set to maintain a specific relation toone another (i.e. a 20% change in outside air corresponds to a 20%change in filtration). As a further alternative, the percentage changein outside air may be determined by the PID algorithm or the fuzzy logicand, thereafter, the percentage change in filtration (%Δcfm_(FLTR)) maybe automatically obtained based on a known ratio between the percentagechanges %ΔOA and %Δcfm_(FLTR). This ratio may be calculated periodicallybased on measured values for the system. This percentage change wouldidentify an incremental amount of outside air Δcfm_(OA) which wouldprovide an amount of dilution within the building equivalent to a changein the amount of filtration Δcfm_(FLTR). For instance, a 10% increase inoutside air may be equivalent in dilution to a 5% increase in filtration(such as when the outside air is average quality). Divergently, a 10%increase in outside air may be equivalent to a 20% increase infiltration when the outside air is of high quality.

FIG. 5 illustrates the processing sequence which may be undergone toobtain this equivalency ratio. At preset points throughout operation,such as at a start up time in each morning, the controller establishesan initial equivalency ratio of 1 to 1 and resets the outside air damperto admit a minimum amount of outside air and the filtration damper toadmit a desired percentage of mixed air (step 300). The percentage towhich the filtration damper is opened may be any initial desiredpercentage, such as 20% and the like. At start up, the controller 29also reads the return, outside and discharge air qualities andenthalpies. These qualities and enthalpies are stored in memory as the"previous" qualities. Once the damper is open by the predefinedpercentage, the conditioning unit is operated for a fixed interval oftime (15 minutes and the like). Thereafter, the controller reads the newoutside, return and discharge air qualities and enthalpies (step 304).Next, the controller calculates the percentage improvement in the returnair quality and stores this improvement within the equivalency ratioupon the side thereof corresponding to filtration. This percentageimprovement is based upon the previous air quality reading for thereturn air and the current return air quality reading (measured in steps301 and 304, respectively). Thereafter, the outside air and filtrationdampers are readjusted, such that the filtration damper is completelyclosed and the outside air damper is increased by a percentage (abovethe minimum acceptable amount of outside air) equal to the percentagechange in the filtration damper (step 308). In this manner, the outsideair damper is opened by an amount, such as 20%, equal to the amount thatthe filtration damper was opened in step 301. Next, the controlleroperates the conditioning unit at the current settings for thepredetermined fixed period of time (step 310) and re-reads the outside,return and supply air qualities and enthalpies (step 312). Next, in step314, the percentage improvement in the return air is calculated andstored within the equivalency ratio on the side thereof corresponding toan increase in outside air.

By way of example, assume that the air quality is measured on a scale of0 to 100 with 0 representing very good quality and 100 representing verybad quality. Further assume that the equivalency ratio is initially setto 1:1 and that the filtration damper is initially set to admit 20% ofthe mixed air therethrough (in step 301). Further assume that the returnair quality at steps 300, 304 and 312 are read to equal 60, 40 and 10,respectively. Based upon these return air quality readings, thecontroller determines that a filtration increment of 20% achieves a 20point improvement in the return air quality. Thus, this improvement isplaced upon the side of the equivalency ratio corresponding tofiltration. The controller further concludes, in step 314, that a 20%increase in outside air provides a 30 point improvement in return airquality. Thus, the controller stores the 30 point improvement upon theside of the equivalency ratio corresponding to outside air (i.e., theequivalency ratio equals 20:30 or 2:3).

Thereafter, when the incremental increase in outside air %ΔOA isdetermined based upon a PID algorithm, the controller utilizes theequivalency ratio to determine an equivalent percentage change in thefiltration damper %Δcfm_(FLTR). If the equivalency ratio is 2:3 F:OA;then a 30% increase in the outside air damper corresponds to a 20%increase in the filtration damper. The controller may recalculate theequivalency ratio at periodic times throughout the day.

The controller may further initiate each day's operation with anequivalency ratio for a similar period of time for a previous day, or asimilar period of time for the same day of the previous week, thusutilizing the same equivalency ratio for each Monday morning rush hourand the like.

As a further alternative, when in a non-steady state application, theimprovement achieved by the filter may be instantaneously calculated byopening the filter damper to filter 100% of the mixed air. Thecontroller reads the quality of the filtered air and uses this readingas the fixed air quality leaving the filter. This fixed air quality isused in the equivalency ratio when determining the %Δcfm_(FLTR) that isequivalent to the %Δ in outside air.

For example, if the air passing through the filter has an effectivecleanliness rating of 20 points on a VOC or other air quality sensorscale which rates air cleanliness on a rating of 0 for perfect airquality and 100 for extremely dirty air quality, and the current outdoorair has a rating of 10, the controller would require twice as large ofan increment of air to pass through the filter to equal an increment ofdilution through the use of outside air.

The controller determines the incremental amount of outside air to beadded or the incremental amount of mixed air to be filtered based upon acontinual process for measuring the difference between the return airquality rating and the desired return air quality rating.

Generally, the "base" increment for increases in outside air orfiltration is calculated on the ability of the filter to removecontaminants. This base increment can be established by opening the facedamper to filter 100% of the mixed air and measuring the air quality ofthe supply air after it passes through the filter. This quality iscompared with the air quality of the mixed air prior to filtration toachieve a filtration efficiency. This filtration efficiency is used toobtain the base increment with which the filter is adjusted. Thecontroller can be programmed to reestablish the base increment at anypre-timed interval, such as daily, weekly or monthly.

FIG. 6 illustrates a look up table which may be utilized to calculatethe cost associated with a given amount of air flow through the filterand a given amount of outside air. Within FIG. 6, lines 400-403represent linear curves identifying a relation between cost and outsideair flow. Each of lines 400-403 correspond to a different enthalpydifferential between the return and outside air. For instance, if theenthalpy differential equals 10, the cost analysis associated withheating and cooling such air would be based upon line 402. Similarly, ifthe enthalpy differential equals 1, the cost associated with heatingadditional outside air would be calculated based upon the linear curve400.

Curves 405-407 represent the relationship between cost and the air flowthrough the filter. Each of curves 405-407 correspond to a differentfilter type and/or characteristic. For instance, as a filter becomesplugged up, the pressure differential thereacross increases, therebyadjusting the cost versus cfm_(FLTR) curve associated therewith.Similarly, different types of filters exhibit different pressure dropsthereacross and similarly have different cost curves. The filter costcurve 405-407 may be calculated by measuring the static pressure dropacross the filter at a given flow of cfm therethrough (i.e., set cfm).In the example of FIG. 6, line 405 corresponds to the cost associatedwith a static pressure of one inch across a filter when the set cfm ispassed therethrough. Similarly, lot curves 406 and 407 correspond tostatic pressure drops of 1.8 and 0.8 inches at the set cfm level,respectively. The filter cost curves include replacement cost and energycost. Thus, the curve becomes steeper as the replacement cost increases.

As previously noted, the controller may initially obtain the equivalencyratio by setting the filter to filter 100% of the incoming air and byreading the filtered or discharge air quality Q-FLTR and the outside airquality Q-OA. When the filter is open 00%, the discharge air qualityrepresents the best air quality capable of being output by the currentfilter. The equivalency represents a relation between the best airquality achievable by the filter (Q-FLTR) and the outside air quality(Q-OA) (i.e., (Q-FLTR)/(Q-OA). This equivalency ratio may be used withinFIGS. 3A-4B to obtain the percentage change in outside air %ΔOA when thepercentage change in filtration %Δcfm_(FLTR) is generated by a PIDalgorithm or fuzzy logic. Similarly, the equivalency ratio may be usedto calculate the percentage change in filtration %Δcfm_(FLTR) once thePID/fuzzy logic obtains a percentage change in outside air %ΔOA. Theequivalency ratio may also be used in the follow alternative system forcalculating costs for heating, cooling and filtration.

FIG. 7 illustrates an alternative method, such as used in steps 112,122, 512 and 522, for calculating the filtering costs (replacement andfan energy) and the energy costs for maintaining the enthalpy level ofthe mixed air at the setpoint. The method of FIG. 7 compares absolutecosts, not the incremental costs. The absolute filtration costs equalsthe total costs associated with filtering (i.e., cfm_(fltr) ) includingthe costs associated with the percentage change Δcfm_(fltr) infiltration plus the costs associated with the current mixed air beingfiltered cfm_(CFLTR). The absolute heating/cooling costs equals thecosts associated with the percentage change %ΔOA in outside air plus thecosts associated with the current outside air flow cfm_(COA).

In this alternative method, when processing flow reaches step 112 and138, control passed to FIG. 7. First, the controller gets the currentequivalency ratio (Q-FLTR)/(Q-OA), and the percentage change infiltration (as obtained in step 111) %Δcfm_(FLTR). The controller alsoobtains the pressure drop ASP across the filter at the design or maximumair flow. The design pressure drop ASP may be obtained during a start uproutine or periodically by setting the filter damper to a design settingand measuring the pressure drop thereacross.

Next, the controller obtains the enthalpy differential Ah between theoutside air enthalpy and the return air enthalpy. The percentage changein filtered air flow %Δcfm_(FLTR) and the current filtration air flowcfm_(FLTR) are used in step 406 to calculate the total air flowcfm_(FLTR) which will pass through the filter once the filter damper isadjusted by the preferred percentage change %Δcfm_(FLTR). At step 408,the controller uses the look up table to obtain a total costs associatedwith filtration if the filter is adjusted to filter cfm_(FLTR) of air.The total filtration cost is obtained (as illustrated in FIG. 6) byusing pressure drop ASP to locate the proper filtration cost curve405-507 and by using the total filter air flow cfm_(FLTR) to locate thecorresponding cost upon the curve. For example, assume the ΔSP=0.8 andthe cfm_(FLTR) equals 1000. Thus, the look up table of FIG. 6 will beaccessed and filtration curve 407 will be used to provide a totalfiltration cost $_(FLTR1) (at point 450). Divergently, if the ΔSPequaled 1 inch, filtration curve 405 would be used and the filtrationcost would equal $_(FLTR2) (at point 452).

Next, the controller obtains the appropriate enthalpy line Δh_(prop) tobe used based on the equation Δh/(Eq Ratio); wherein Δh represents theenthalpy value obtained in step 402 and Eq Ratio represent theequivalency ratio obtained in step 400. The Δh_(prop) identifies anenthalpy line that has been proportionally adjusted via the equivalencyratio to be equivalent to the filtration curve. Next, the controlleraccessed the lookup table again to obtain the total cost for heating theoutside air cfm_(OA). TO do so, the controller obtains the properenthalpy line 400-403 based on the enthalpy difference Δh_(prop). Thecontroller then locates the cost point upon the line Δh_(prop) for thecurrent flow of filtered air cfm_(FLTR).

For instance, if the outside and return air enthalpies equal 30 and 10respectively, and the Eq Ratio is 2:1, the enthalpy difference Δh_(prop)will equal 10 (i.e., (30-10)/2). Thus, line 402 is used within thelookup table. If 1000 cfm of filtered air are used, the cost of usingoutside air will equal $_(HEAT).

In step 414, the $_(HEAT) cost for heating the outside air is cheaperthan the cost of filter 1000 cfm $_(FLTR1). The foregoing analysis isequally applicable to calculating the cost of cooling.

Alternatively, when the percentage change in outside air AOA is obtainedin step 109 via the PID algorithm, the process of FIG. 7 may be repeatedby removing the variables %Δcfm_(FLTR), ΔSP, Δh, cfm_(FLTR), andΔh_(prop) and replacing these variables with the variables %ΔOA, Δh,ΔSP, cfm_(OA), and ΔSP_(prop), respectively. The cost calculates insteps 408 and 412 are interchanged. Thus, the percentage change inoutside air and the change in enthalpy are obtained in step 402. Thechange in static pressure is obtained in step 404. The new total outsideair is obtained in step 406. The enthalpy cost of maintaining theenthalpy of the mixed air at the set point is obtained from the enthalpylines in step 408. The proportional change in static pressure ΔSP_(prop)is obtained in step 410 (i.e., ΔSP/(Eq Ratio)). Finally, the filteringcost is obtained in step 412.

During operation, the controller is able to set the filter to pass theset amount of cfm and measure the static pressure drop thereacross. Fromthis measurement, the controller is able to calculate the correspondingcurve 405-407 to be used. The controller further calculates the costcurve for outside air 400-403 based upon the enthalpy of the outsideair. Once these curves have been identified, the controller is able touse a look up table containing the corresponding cost values for a givencfm of outside air or a cfm of filtration.

As a further alternative, a flow measurement sensor may be included andthe controllers operations based thereon to calculate the optimalsequence of damper settings based on the above variables plus filteringcosts (i.e., pressure drop, fan horsepower and filter replacement cost),versus energy costs of heating or cooling the added air brought in fordilution.

Optionally, air quality control module which may be implemented with thecontroller. The air quality control module includes a CO₂ monitor and aVOC monitor. A pump is included which draws air in through an inlet portand discharges such air via a discharge port. First and second solenoidvalves are connected in series with the inlet port of the pump. Thesolenoid valves each include one normally closed inlet port and onenormally open inlet port and one discharge port. The first inlet portsare connected to a ventilation tube which delivers outside air to theport. The second inlet ports are connected to a conduit which deliversreturn air to the port and to a conduit which draws upon supply airwithin the intake chamber. The discharge port of the secondary solenoidvalve delivers its output to the inlet port of the first solenoid valve.The discharge port of the first solenoid valve delivers its output tothe inlet port upon the pump.

The first and second solenoid valves are controlled, via the controller,to selectively deliver one of the supply air, return air and outside airto the pump. This quantity of air is sampled to determine the level ofcontaminants therein including the CO₂ level and the VOC level.

During operation, the air sampling pump runs continuously whenever theunit is turned on. The sampling pump begins 30 minutes before anyscheduled start up time of the conditioning unit. A selector within thecontroller selectively operates the solenoid valves in order toselectively deliver supply, return and outside air separately to themonitoring system. Each air source is sequentially read and thecontroller logs outside air quality (i.e., CO₂ level and VOCcontaminants). The controller sequentially reads and logs outside airquality for selected gaseous contaminants by energizing the selectedsolenoid valves and reading the air quality sensors after allowing forthe sampling chamber to be flushed. The controller reads the air qualityonce the sensor has stabilized (which typically occurs at 10 minuteintervals). When no solenoids are energized, the controller samples theoutdoor air. If the first solenoid is energized, the return air issampled. When both solenoids are energized, the supply air is sampled.The data read from the air quality sensors in the module is read by thecontroller and used for IEQ control.

In addition, the inventive system may optionally include a ventilationcontrol module which guarantees that the return and outside air dampersare set to ensure that the minimum amount of outside air is forced intothe intake chamber, even during low system air flow. The ventilationcontrol module maintains this minimum outside air flow rate byoverriding the return air damper as necessary to force additionaloutside air into the system.

An integral face and bypass coil section may be provided to assurefreeze protection when the system requires a high percentage of outsideair, while the outside air temperature is quite low.

The selector and controller sample and log outside, return and supplyair qualities so that the integrated controller 29 can determine, basedon operating cost and current air quality, whether the best course ofaction is to increase or decrease the amount of outdoor air, and whetherto direct the air through or around the gas phase/particulate filtrationsystem.

As is clear from the foregoing description the inventive system allowthe user to achieve optimum air quality by comparing the costsassociated with filtration with the costs associated with introducingadditional outside air to dilute the contaminates within the building.The filtration costs may include power costs and replacement/filter wearcosts. The user may adjust the relative weight of each factor upon thedetermination depending upon which of power costs and replacement costsis of more concern. For instance, if the power consumption duringfiltration is cheap or free this factor would weight very little in theforegoing determination. Alternatively, if the filter replacement costsare of more concern, this factor could be set to weight more heavilywithin the determination.

As explained above, the preferred percentage changes in air flow throughthe filter damper %cfm_(FLTR) and the outside air damper %cfm_(OA) maybe set at fixed values for each iteration through the processingsequence of FIGS. 3A-4B. However, these percentage changes may be variedbetween iterations depending upon the air quality of the outside andreturn air and the relation therebetween. The outside air may have badquality during rush hour (if the building is located in a high trafficarea), while the outside air may have good quality at night. In suchsituations, during rush hour 40% of additional outside air may be neededto achieve the same result as 10% of additional outside air duringnon-rush hour or at night. In this situation, the outside air damper isadjusted based on the outside air quality and the filter damper isadjusted based upon the filtering efficiency of the filtration system.

For instance, for the following example, assume that the outside andreturn air may have good, average or bad quality. When the outside airhas good quality and the return has bad quality, the outside air dampermay be opened during a single iteration by a small percentage change(10%). However, if the outside air quality is average while the returnair quality is bad, the outside air damper may be opened during a singleiteration by a medium percentage (25%). Further, if the outside airquality is bad (but better than the minimum standard), and the returnair quality is worse than the minimum standard, the outside air dampermay be opened during a single iteration by a large percentage (40%).Thus, the amount of variation in the outside air damper may bedetermined based on the outside and return air qualities.

The percentage change in the filter damper may also be obtained based onthe return air quality and the filter efficiency. This independentchange accounts for the fact that a filtration system may improve theair quality of a given volume of air at a different rate than an equalvolume of additional outside air. For instance, 10% of the mixed air mayneed to pass through the filter to create the same supply air quality aswould be achieved if an additional 30% of outside air were added.

It is to be understood that the constant K adjusts for air densityvariation and the that the foregoing equations are based on standard airdensity. The controller will compensate, within every equation, for anyvariations from the standard air density to correct for altitude andtemperature.

The inventive system may optionally provide for such control byincluding exhaust relief and minimum outside air control to maintainminimum positive building pressure.

Optionally, once the actual total preferred amount of outside aircfm_(POA) is obtained, the controller may next calculate the amount ofoutside air that can be introduced through the damper 30 withoutnecessitating the use of the heating coil. The calculation for cfm_(MOA)is optional since, in the above example, the economizer cycle sets theoutside air flow to equal the maximum outside air flow usable whilemaintaining the mixed air temperature setpoint. In such a situation, theoutside air flow measured between steps 106 and 110 would equal thismaximum. However, if the economizer cycle does not set the outside airflow to this maximum cfm_(MOA) then the maximum must be calculated basedon the equation

    (cfm.sub.MOA)(t.sub.OA)+(cfm.sub.MA -cfm.sub.OA)(t.sub.RA)=(cfm.sub.MA)(t.sub.MA),

where cfm_(MOA) represents the maximum cfm of outside air that may beintroduced without using the heating coil capacity to maintain thesupply air temperature at or above a predefined set point t_(SA). Inthis equation, the temperature of the supply air t_(SA) is set to equalthe predefined set point, such as 50° F. In this optional situation, themaximum outside air flow would be subtracted from the total preferredoutside air flow before determining whether the heating coil capacity issufficient in step 110.

Optionally, the energy calculations performed above in connection withheating outside air may be based upon enthalpy, instead of temperature.This enthalpy calculation is used when, in the winter, it is desirableto increase the humidity of the supply or discharge air. Thus, outsideair admitted to the system must be heated and increased in humidity.Accordingly, the calculations performed in steps 110 and 112 aremodified to account for enthalpy changes and enthalpy differentialsbetween the enthalpy set point h_(MA-SET) and the resulting mixed airenthalpy h_(MA-RES). These enthalpy calculations may be conducted in amanner similar to that performed in step 138 to calculate the enthalpyvariation corresponding to a cooling operation. This enthalpycalculation will account for the cost of humidifying dry outside air.

From the foregoing it will be seen that this invention is one welladapted to attain all the ends and objects hereinabove set forth,together with the other advantages which are obvious and which areinherent to the invention.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

I claim:
 1. An air conditioning unit comprising:a housing having aninlet opening and a discharge opening; a filtration system, positionedwithin said housing, for filtering supply air traveling through a firstpassageway between the inlet and discharge openings, said filtrationsystem having means for removing volatile organic compounds from saidsupply air; a fan mounted within said housing for directing inlet airinto the unit through said inlet opening and discharge air, out of saidunit, through said discharge opening; a bypass for routing inlet airthrough a second passageway between said inlet and discharge openingsbypassing said filtration system; and a flow control device forcontrolling a percentage of said inlet air passed through each of saidfirst and second passageways based upon a quality of said inlet air. 2.The invention of claim 1, wherein said flow control device comprises afirst section for regulating flow of said supply air through saidfiltration system in said first passageway and a second section forregulating flow of said supply air through said second passagewaybypassing the filtration system, said first and second passagewaysreceiving 100% of the inlet air.
 3. The invention of claim 1, includingan air quality detector operably coupled with said flow control device,said flow control device automatically adjusting an amount of inlet airpassed through each of said first and second passageways in response tosupply air filtration requirements determined by said detector.
 4. Theinvention of claim 1, wherein said inlet opening includes return andoutside air ports for receiving return and outside air, respectively,said flow control device further including flow control dampersproximate said return and outside air ports for controlling an amount ofair passed through associated ports.
 5. The invention of claim 1,wherein said filtration system comprises at least one of a high capacitymechanical filter, high capacity adsorptive filtration system and a gasphase filter.
 6. The invention of claim 1, further comprising means fordetermining an incremental filtration cost corresponding to anincremental increase in an amount of inlet air passed through said firstpassageway and through said filtration system, said flow control deviceadmitting said incremental increase of inlet air through said filtrationsystem when said incremental filtration cost falls below a predefinedlevel.
 7. The invention of claim 1, said inlet air comprising outsideand return air, said unit further comprising means for determining anincremental energy cost corresponding to an incremental increase in anamount of outside air admitted to form said inlet air, said flow controldevice maintaining said percentage of inlet air passed through each ofsaid first and second passageways constant and admitting saidincremental increase of outside air when said incremental energy costfalls below a predefined level.
 8. The invention of claim 1, said inletair comprising outside and return air, said unit furthercomprising:means for determining a dilution preferred percentage changein an amount of outside air admitted and a filtration preferredpercentage change in an amount of inlet air passed through said firstpassageway and said filtration system, and means for adjusting said unitbased upon one of said filtration and dilution preferred percentagechanges depending upon a quality of said outside air.
 9. An airconditioning unit comprising:a housing having an inlet opening and adischarge opening; a filtration system, positioned within said housing,for filtering supply air traveling through a first passageway betweenthe inlet and discharge openings; a fan mounted within said housing fordirecting inlet air into the unit through said inlet opening anddischarge air, out of said unit, through the discharge opening; a bypassfor routing inlet air through a second passageway between said inlet anddischarge openings bypassing said filtration system; a flow controldevice for controlling a percentage of said inlet air passed througheach of said first and second passageways based upon a quality of saidinlet air; and means for determining an incremental filtration costcorresponding to an incremental increase in an amount of inlet airpassed through said first passageway and through said filtration system,said flow control device admitting said incremental increase of inletair through said filtration system when said incremental filtration costfalls below a predefined level.
 10. An air conditioning unitcomprising:a housing having an inlet opening and a discharge opening; afiltration system, positioned within said housing, for filtering supplyair traveling through a first passageway between the inlet and dischargeopenings; a fan mounted within said housing for directing inlet air,comprised of outside and return air, into the unit through said inletopening and discharge air, out of the unit, through the dischargeopening; a bypass for routing inlet air through a second passagewaybetween said inlet and discharge openings bypassing said filtrationsystem; a flow control device for controlling a percentage of said inletair passed through each of said first and second passageways based upona quality of said inlet air; means for determining a dilution preferredpercentage change in an amount of outside air admitted and a filtrationpreferred percentage change in an amount of inlet air passed throughsaid first passageway and said filtration system; and means foradjusting said unit based upon one of said filtration and dilutionpreferred percentage changes depending upon a quality of said outsideair.
 11. An air conditioning unit comprising:a housing having an inletopening and a discharge opening; a filtration system, positioned withinsaid housing, for filtering supply air traveling through a firstpassageway between the inlet and discharge openings; a fan mountedwithin said housing for directing inlet air, comprised of outside andreturn air, into the unit through said inlet opening and discharge air,out of the unit, through the discharge opening; a bypass for routinginlet air through a second passageway between said inlet and dischargeopenings bypassing said filtration system; a flow control device forcontrolling a percentage of said inlet air passed through each of saidfirst and second passageways based upon a quality of said inlet air; andmeans for determining an incremental energy cost corresponding to anincremental increase in an amount of outside air admitted to form saidinlet air, said flow control device maintaining said percentage of inletair, passed through each of said first and second passageways, constantand admitting said incremental increase of outside air when saidincremental energy cost falls below a predefined level.
 12. An airconditioning unit comprising:a housing having an inlet opening and adischarge opening; a filtration system, positioned within said housing,for filtering supply air traveling through a first passageway betweenthe inlet and discharge openings; a fan mounted within said housing fordirecting inlet air into the unit through said inlet opening anddischarge air, out of the unit, through the discharge opening; a bypassfor routing inlet air through a second passageway between said inlet anddischarge openings bypassing said filtration system; and a flow controldevice for dividing flow of said inlet air between said first and secondpassageways in proportion to a seused quality of said inlet air, whereinsaid first passageway receives a first percentage of said inlet air andsaid second passageway receives a second percentage of said inlet air.13. The invention of claim 12, including an air quality detectoroperably coupled with said flow control device, said flow control deviceautomatically adjusting an amount of inlet air passed through each ofsaid first and second passageways in response to supply air filtrationrequirements defined by said detector.